What term describes a slight depolarization of the postsynaptic membrane that brings it closer to the threshold for an action potential?

• A. EPSP
• B. IPSP
• C. Graded potential
• D. Resting potential

Answer: (A)

A slight depolarization of the postsynaptic membrane toward threshold is an excitatory postsynaptic potential. When a neurotransmitter binds to receptors, it often opens cation channels that let positively charged ions (like Na+ or Ca2+) enter, pulling the membrane potential up toward the threshold. This depolarization is a graded potential, meaning its magnitude depends on how strong the stimulus is, and it can summate with other inputs. If enough excitatory input accumulates to reach the threshold, voltage-gated Na+ channels open and an action potential fires. This differs from an inhibitory postsynaptic potential, which would hyperpolarize and move the membrane further from threshold, and from resting potential, which is just the baseline voltage when the neuron is not stimulated.